1. Field of the invention
The present invention relates to a method and to a device intended for permanent seismic monitoring of an underground zone.
2. Description of the Prior Art
It is for example well-known to test the filling of an underground reservoir intended for natural gas storage at successive times, by means of a seismic system comprising a vibrator or an impulsive source for applying impact pulses to the ground surface and a reception device comprising alignments of elastic wave pickups positioned at the surface or in a well and coupled with the ground. As gas injection significantly alters the sound velocity, the density and therefore the reservoir formation, it is possible to determine the variation of the time interval on the seismic records between two reflectors, one being positioned above, the other below the reservoir. Filling evolution can also be followed by measuring the amplitude variation of the waves reflected near to the layers of the reservoir, at successive filling times.
U.S. Pat. Nos. 4,775,009, 4,986,350, 5,181,565 and 5,481,501 and notably, French Patent 2,688,896, describe systems comprising one or more service sets installed permanently in wells connected to the surface by means of one or more transmission channels. These service sets can be seismic receivers and possibly one or more seismic wave sources set in wells. The receivers or emitters can be placed outside casing pipes and coupled with the surrounding formations by the cement injected in the annulus around these pipes once set. The arrays of receivers can also be associated with a tubing lowered in a well and coupled with the formations surrounding the well if they are pressed against the wall of the well directly or by means of a casing pipe for example.
French Patents 2,703,457 and 2,703,470 and U.S. Pat. No. 5,243,562 describe seismic monitoring methods for underground reservoirs using arrays of receivers placed in wells or of seismic sources placed at the surface or possibly in these wells.
Seismic subsurface exploration is generally performed by coupling with the ground seismic sources or receivers according to various combinations where the sources and/or the receivers are positioned at the surface or in the neighborhood thereof, or in one or more wells through the explored formation.
A series of seismic emission-reception cycles are carried out by changing each time the location of the seismic source in relation to the axis of the well where the arrays of receivers are installed, according to a technique known as &lt;&lt;walk-away&gt;&gt;, and by recording the arrivals at receivers R1 to Rn as a function of the propagation time t.
Monitoring of oil-bearing reservoirs or of gas storage reservoirs generally implies long-lasting operations because the variations to be observed are relatively slow.
U.S. Pat. No. 5,724,311 of the Assignee describes a permanent monitoring system that obtains a perfect reproducibility of the operating conditions in repetitive seismic monitoring operations in an underground zone crossed by at least one well or borehole, and notably an underground gas storage reservoir. This system comprises one or more arrays of seismic receivers (buried, at the surface or placed in one or more wells), several repetitive seismic sources (buried or at the surface), all permanently installed, and a permanent link network for selective power supply to these sources. A central station selectively remote-controls each of these sources and records the seismic signals coming from the underground zone in response to the seismic waves transmitted selectively in the ground by the sources.
This array of permanently installed sources, whose coupling with the surrounding formations remains stable, and this at least partly buried supply network, whose surface coverage area is reduced, allow to carry out a whole series of long-lasting seismic monitoring operations under stable operating conditions without risks of incompatibility with the activities of the production site.